JPA: how to persist many to many relation

I have these 2 entity with many to many relationship.

@Entity
public class User {

    @ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    private List<User> users = new ArrayList<User>();
}

@Entity
public class Language {
    @ManyToMany(mappedBy = "languages")
    private List<User> users = new ArrayList<User>();
}

I already have 20 languages saved in my language table. Now, I want to create a user and give relate that user with first language in the language table. So I did something like this

    Language selectedLanguage = languageService.findById(1);
    stammdaten.getLanguages().add(selectedLanguage);
    stammdatenService.save(stammdaten);

But this gives me error org.hibernate.PersistentObjectException: detached entity passed to persist: com.example.outgoing.Entity.Language. So how can I save this many to many relation. One thing to note here: I don't want to add new language. I want to add new user with already created languages.